The present invention relates to a method and apparatus for mechanical power transmission and position or timing control by means of a belt or chain driven by an arrangement of a shaft and sprocket or pulley in which the shaft is coupled to the sprocket or pulley by means of a tapered bushing so as to ensure substantially concentric motion of the components and to allow phasing of the shaft-mounted components.
Typical torques employed in both mechanical power transmission and motion control applications are often such as to cause significant problems with torque transmission between a shaft and any gears, pulleys or sprockets that are intended to be driven by the shaft. Various methods that are employed to achieve the requisite coupling of torque include set screws, pins, keys, flattened shafts, flanged bushings and clamping couplings of various sorts.
Additionally, in power transmission applications employing sufficiently large shafts, tapered bushing assemblies are known for coupling rotating shafts to sprockets so that rotary motion of the shaft may be transmitted to the sprocket. In the power transmission art, the coupling of substantial torque between a shaft and a hub by a bushing gives rise to requirements such as keys, flanges coupled to the exterior face of the sprocket, and/or large surface areas of interface between the bushing and the shaft and hub respectively. The use of large surface areas of interface is based on the proportionality of static friction, for transmission of shear forces, to the contact area between the surfaces. A typical prior art hub and bushing structure is now described with reference to FIGS. 1 and 2.
Numeral 10 designates generally a sprocket or pulley for driving a chain or belt (not shown), the sprocket having an outer surface 12 and a hub 14. The subject of the present discussion is the coupling of torque between a rotatable shaft 20 of constant diameter and hub 14 by means of a bushing 22. In the power transmission art, the coupling of substantial torque between shaft 20 and hub 14 via bushing 22 is commonly accomplished by using keyways machined into interior bore 46 of the bushing and outside diameter 44 of the shaft, with a solid square key 48 inserted into the machined spaces. The use of square keys and keyways for obtaining maximum torque is described in ANSI Standard B17.1, Keys and Keyseats, which is incorporated herein by reference. In this way, the torque that may be coupled by means of shear forces exerted on the keyways increases as the size and strength of the key and keyways increase. Alternatively, torque may be transmitted by means of one or more connectors or pins inserted through an annular flange of the bushing and a side face of the hub. When an annular flange is employed, the maximum diameter of the bushing exceeds the inside diameter of the hub.
Referring now to FIG. 2, a cross-sectional view is shown of the prior art sprocket 10 and bushing 22, with the section taken on line 2xe2x80x942 of FIG. 1. Mating tapered surfaces of the outside 24 of bushing 22 and the inside 26 of hub 14 are shown, while internal surface 27 of bushing 22 is shown to be straight and parallel with the surface of shaft 20. Bushing 22 is continuous throughout with the exception of a radial slot 28 of sufficient width to permit the bushing to contract during installation and to grip the shaft firmly. Tapered surfaces 24 and 26 of the bushing and hub, respectively, are adapted to slide relative to one another as the sprocket is assembled onto the shaft and the bushing is secured in place within hub 14 In a power transmission application, the scaling relationships that are known to hold require that the surface area in contact between bushing 22 and hub 14 must increase at least as fast as the torque required to be transmitted.
In order to insert bushing 22 into hub 14 of sprocket 10, a force is required to overcome the sliding friction developed across the entire contact surface area between the bushing and the hub. A similar sliding friction must be overcome to disengage the bushing from the hub. The sliding friction that must be overcome is proportional to the surface area and thus, as discussed above, to the torque capacity of the bushing.
Since the objective of power transmission applications is typically the transmission of a maximal amount of torque for given geometrical and material constraints, it has been deemed desirable, in the prior art, to maximize the contact surface area (xcx9cT) between the bushing and hub. This makes it difficult, however, to overcome the frictional hurdles both for insertion and disengagement of the bushing. Therefore, the prior art teaches that a taper with an included angle of at least 8xc2x0 (equivalent to a taper angle xcex1=4xc2x0) is placed on external surface 24 of bushing 22 and internal surface 26 of hub 14 in order to reduce the difficulty in releasing the bushing. Typically, a threaded hole 76 (shown in FIG. 1) is provided for insertion of a jack-screw so that force may be applied for release of the bushing.
Engagement of the bushing, moreover, requires that both the sliding friction across the contact surface as well as the lateral component due to the taper must be overcome. In the prior art, this has been accomplished by use of at least two screws 30 and 32, along with matching tapped threading on the hub, for drawing the bushing into the hub. Consequently, clearance must be provided in the placement of the components for tightening the bushing and for removing the bushing using a jackscrew. An additional concern is the square entry of the bushing upon insertion; since the contact surface area is large (scalingxcx9cT), the bushing is prone to cocking upon insertion if care is not taken in driving the symmetrically placed screws.
Additionally, the scaling relations of the prior art power transmission applications dictate that the annular width w (shown in FIG. 1) of bushing 22 must scale substantially as the bushing face thickness t since txcx9cTxc2xd. The annular width, however, is half the difference between the outer diameter and inner bore of the bushing. Therefore, wxcx9cdpxe2x88x92dsxcx9cxcex3Txc2xdxe2x88x92xcex6T⅓, where xcex3 and xcex6 are coefficients of proportionality. The annular width dimension, w, typically scales as Txc2xd in the limit of large torque, which is the limit of interest in prior art power transmission applications. While the foregoing scaling relations are not intended to be definitive or descriptive of all bushings available in the prior art, they are intended to illustrate the nature of design considerations governed primarily by maximizing torque capacity as taught in the power transmission art.
In accordance with a preferred embodiment of the present invention, there is provided a method for applying a bushing to improve the concentricity of a coupling between a shaft and a hub having a central bore. The bushing has an interior bore for surrounding the shaft and exactly one exterior slot in a direction parallel to the interior bore for matching a corresponding slot in the hub. Each matched part of the slot defines an opening for receiving a set screw. The method includes driving the set screw into the exterior slot in such a manner as to draw the bushing into the bore of the hub, thereby coupling the shaft to the hub.
In accordance with alternate embodiments of the present invention, there is provided a method for improving the concentricity of a coupling between a shaft and a hub of the type where the shaft has a diameter less than 0.5 inch. The method has the steps of providing a split tapered bushing, where the bushing has an inner bore of less than 0.5-inch diameter and an outer diameter smaller than the inner diameter of the hub, and using the bushing to couple the shaft to the hub.
In accordance with a further aspect of the present invention, there is provided a device for coupling a shaft to a hub, where the hub is of a first material and has a first surface finish. The device has a split bushing with an exterior surface and an interior bore of diameter less than 0.5 inch and a depth. The bushing has an annular width defined as a normal distance between the interior bore and the exterior surface. The device also has at least one transversely located exterior slot for matching a corresponding slot in the hub, each matched part of the slot defining an opening for receiving a set screw. In accordance with alternate embodiments of the present invention, the device may have exactly one transversely located exterior slot, and the exterior surface of the bushing may be tapered at an angle less than three degrees with respect to the axis of the shaft. The bushing may be of a material or surface finish distinct from that of the hub.